sub usage {
  my $text = <<USAGE;
$0 [ -i | -c | -t ] directory [, directory ... ]
Frobnicate all the files in the specified directory/ies
-t   test only, and warn of discrepancies
-i   inclusive: unknown files ignored
-c   conclusive: unknown files deleted
(default is to defrobnicate unknown files)
USAGE
  die "$text\n@_\n" if @_;
  print $text;
  exit 0;
}